Is it possible to create a compositor node with PyNodes that outputs camera clip_start and clip_end?
Not sure if this is possible with PyNodes, Seems like the sort of thing you'd use Drivers for.

You can use node settings to drive these values too,
Add a driver to the cameras clipping, and set the driver to point to a scene property, eg: node_tree.nodes["Mix"].inputs[0].default_value



And the other way around you can do too, of course. Add a driver to a value node, and set the driver to point to the camera property like clip_sta and clip_end.